Self-oscillating resonant power converters, such as commonly used in compact fluorescent lamp ballasts, for example, typically operate by deriving a transistor switching waveform from one or more windings magnetically coupled to a resonant inductor. U.S. Pat. No. 5,965,985 of Nerone describes a circuit for such a ballast that allows control of the output to a load in order to provide lamp dimming capability. U.S. Pat. No. 5,965,985 describes the control of a self-oscillating ballast by effectively clamping the voltage excursion across an inductor. The effect is to control the reactance of the inductor clamp combination. A similar method of achieving such a result is to vary the effective reactance of a reactive element using a variable resistance coupled in series or parallel therewith. The variable resistance is typically implemented with an active element, e.g., a transistor, wherein the effective resistance across two terminals is a continuous function of the magnitude of the control signal. The applied control signal is also continuous and has a maximum frequency component that is substantially less than the switching frequency of the converter.
It is desirable to implement control circuitry, such as of a type described hereinabove, on an application specific integrated circuit (ASIC) in order to achieve low complexity and cost. It is furthermore desirable to implement as much of the control circuitry as possible in digital form. Unfortunately, the control method described hereinabove inherently requires an analog, continuous signal. Hence, a digital approach, when combined with the control method described hereinabove, requires a digital-to-analog converter to generate the control signal, adding to the complexity of the system. In addition, the analog approach may result in significant power dissipation in the control element, making it impractical to integrate on an ASIC chip. These latter drawbacks may be overcome using a switch control waveform synchronized to the converter power switching waveforms, as known in the art, but for a self-oscillating converter, this results in the requirement of a frequency tracking circuit, such as a zero-crossing detector or phase-locked loop. This requirement may substantially increase cost, complexity, and size of the system.
Accordingly, it is desirable to provide a control for a self-oscillating switching power converter using an active control device in a manner that does not require the control switch waveform to be synchronized with the converter switching frequency. It is furthermore desirable that such control device be operated in a digital manner, that is, with two operating states (on and of f and that the control input for the device also be digital. It is furthermore desirable that such a control avoid compromising the response speed of the converter, so that maximum performance may be obtained.
In accordance with exemplary embodiments of the present invention, a self-oscillating switching power converter has a controllable reactance comprising an active device connected in series or parallel with a reactive element, wherein the effective reactance of the controllable reactance and the active device is controlled such that the control waveform for the active device is binary digital and is not synchronized with the switching converter output frequency. Preferably, the active device is turned completely on and off at a frequency that is substantially greater than the maximum frequency imposed on the output terminals of the active device. The effect of such control is to vary the average resistance across the active device output terminals, and thus the effective output reactance, thereby providing converter output control, while maintaining the response speed of the converter.